The recording of the bioelectric responses generated by the retina as a consequence of a visual stimulus perceived by the patient is a remarkably useful tool in order to support the medical operator in diagnostic activity. The aforementioned bioelectric responses are recorded, through suitable electrodes arranged at the level of the conjunctival fornix, the corneal one, or close to the central visual cortex by means of surfaces electrodes (of both eyes in the case of binocular recording), from which an electric biopotential is then obtained. This represents a measurement of the integrity of the visual system (density of cones, rods and cells connected to them, ganglion cells, retinal cells, nerve fibres and visual cortex) or of possible alterations or destructive actions already caused by the various pathologies.
With current techniques (ISCEV Standard for full-field clinical electroretinography—2008 update—M. F. Marmor et al.) ERG and VEP recording is obtained by stimulating the retina and the optical nerve through the dilated pupil, with adaptation of the retina itself to the dark (scotopic condition) or to light (photopic condition). The light stimuli according to this technique are emitted by a lambertian surface having homogeneous intensity (constant luminosity) distributed over the entire visual field of the patient. By effect of the dioptric elements of the eye (cornea and crystalline lens) the light stimulus is distributed through the dilated pupil over the entire surface of the retina. The correction of the photometric measurement of the luminosity values that impact on the eye, scaling them with the actual measurement of the pupil, is carried out according to the Troland unit.
Although widely used, this technique has some limitations, the first of which is the invasiveness due to the insertion of electrodes (the sensors used for recording the bioelectric activity of the retina) in contact with the eye. There is also uncertainty of the measurement due to the difficulty for the patient in keeping his/her eye open during the examination. The possible contamination and transmission of infective diseases deriving from contact with electrodes not properly disinfected should also be mentioned; avoiding this obviously involves slowing everything down when numerous patients must be examined in sequence, since careful cleaning must indeed be carried out between one patient and the next.